The data communications over fiber-optic networks continue to grow exponentially. In order to meet this need, multiplexing technology has been developed to allow multiple scattered data flows to share the same fibers, thus greatly increasing the traffic of each fiber.
In the optical fiber industry, the current research and development focus mainly on DWDM (Dense Wavelength Division Multiplexing). DWDM is one of multiplexing technology in which multiple data channels are assigned to respective wavelengths within a certain operating bandwidth. Data channels are combined with the basic mode (LP01) of single-mode fibers for transmission, and when arriving at their respective destinations, they are returned to separation channels respectively.
In a DWDM-based transmission system, the total capacity within a given amplifier bandwidth is limited by spectral efficiency, and the spectral efficiency is used to describe, at a given data rate, the degree of closeness to which a single wavelength can be spaced for communication purposes when the fibers are subjected to the extreme limitation due to non-linear effects. Although the use of increasingly complex algorithms can improve the spectral efficiency, the decrease in bandwidth gains is brought and the modest improvement can not meet the exponentially-growing bandwidth needs, and the spectral efficiency of DWDM in single-mode fibers will approach its theoretical limits. A promising way to increase the capacity of each fiber is mode-division multiplexing, in which the corresponding multiple optical signal modes guided by fibers are provided. Based on this technology, it can be expected to have the potential to significantly increase the transmission capacity of each fiber, and to break through the limitation of nonlinearity of the DWDM-based system.
At present, the few-mode fiber technology around the world is mainly concerned with the optimization of group delay of fibers, for example, Chinese Patent CN201280019895.2, in which a design of graded-index few-mode fiber for spatial multiplexing is disclosed. However, the above technical solutions are all based on the germanium-doped core region for adjusting the distribution of refractive index of the fiber core region. As germanium-doped quartz has a higher scattering coefficient, a high fiber loss is generated. Moreover, in an application of ultra-long-distance high-capacity optical fiber communications, the attenuation coefficient of germanium-doped graded-index few-mode fibers at 1550 nm is generally above 0.19 dB, and the attenuation coefficient also varies with the change of ambient temperature conditions, resulting in excessive loss which in turn leads to the increase of error codes in the communication system and the increase of relay costs. On the other hand, in short-distance transmission (e.g. in fiber jumper applications), linear polarization modes that are undesired for transmission in fibers need to be dissipated rapidly by means of linear polarization modes, or they will bring about difficulties in signal resolution. Therefore, it becomes a difficult issue for giving consideration to both the low loss in long-distance transmission and the effective attenuation of undesired linear polarization modes in short-distance transmission.